Gas discharge tube sensitive to a.c. signals



c. w. JACOB 3,009,077

GAS DISCHARGE TUBE SENSITIVE TO A.C. SIGNALS Nov. 14, 1961 Filed March12, 1951 FIG. 4.

DE TONA 70R FIG.2.

CARLYLE W. JACOB 3nventor ia' w II IIII lw h bun l h I W IU will \1l/|||| k 7 I l B I; J if nw h M. G R l Z H llllllll ll lllllllllllllllll .llilllulillnnllllllillll attorney Unite tates This applicationrelates to electron tubes and particularly to gas-filled dischargetubes.

Gas-filled grid-controlled discharge tubes are utilized in radioproximity fuzes for ordnance projectiles to initi ate the explosion ofthe bursting charge of the projectile upon the application to the gridof an operating signal of predetermined amplitude and duration. The gridof such a tube has heretofore been conventionally biased considerablymore negative than the starting voltage, i.e., the critical firingvoltage, as a safety precaution against premature detonations caused bydecay during the flight of a projectile of the output voltages of thedeferred action type battery that conventionally provides the grid-biasvoltage. Biasing the grid well beyond the starting voltage diminishesthe sensitivity of the gas-filled discharge device in that a strongfiring signal is required to raise the grid above the starting voltage.

It is an object of the invention to provide a stable gasfilledgrid-controlled discharge device which is sensitive to weak A.C. firingsignals and substantially immune to drifts in D.C. bias voltage.

It is a further object of the invention to provide a stable gas-filledgrid-controlled discharge tube and circuit therefor which exhibitdifferential sensitivity to rapidly varying and to slowly changingsignals.

It is a still further object of the invention to provide a stablegas-filled grid-controlled discharge tube and circuit therefor which canbe fired by weak A.C. signals and which prevents accidental firings dueto drift in grid bias over a range of voltage considerably larger thanthe peak voltage of the A.C. signal required to fire the tube.

Still another object of the invention is to provide such a gas-filledgrid-controlled discharge tube which, without danger of accidentalfiring of the tube, can be safely operated at lower grid bias voltagesthan have heretofore been utilized.

The objects of the invention are attained in a gas-filledgrid-controlled discharge device by providing a balancing fourthelectrode positioned in the electron path through the apertures in thecontrol grid to compensate for changes in electron flow due to drift inthe C bias voltage on the control grid. In operation, a source ofpotential positive relative to the cathode is connected to the balancingelectrode through'a high resistance. A gradual decrease in the cont-r01grid negative bias voltage allows a few electrons to emerge through theapertures in the grid. Practically all of the electrons are collected bythe balancing electrode and flow through the high resistance. Theincreased voltage drop across the high resistance drives the balancingelectrode more negative and thus increases its shielding action. Thebalancing electrode voltage E, is thus dependent inversely upon thecontrol grid bias voltage E For a given anode voltage the tube has acritical flash point function (i.e., control characteristic) of E and Ewherein any combination of such voltages on the non-flash side of thefunction prevents the establishment of the arc. The control grid C biassource and the source of positive potential for the balancing electrodeare selected so that the combination of E, and E determines a point juston the non-flash side of the function, and E balances, i.e., variesinversely with, drift in E to maintain the combination of voltages onthe non flash side of the function, i.e., to limit the anode electronatent current to a value below the minimum required to establish thearc, over a wide range of variation of E Differential sensitivity toA.C. signals and to drifts in control grid bias voltage is obtained byproviding a time delay in the variation of E, when an A.C. signal isimpressed on the control grid. In the preferred embodiment of theinvention, this difference in sensitivity is provided by connecting acapacitance to the balancing electrode in parallel with the highresistance. The capacitance offers a substantially infinite impedance toslow variations in electron flow and presents an impedance path to rapidfluctuations of electron flow which decreases with frequency. Amomentary delay greater than the ionization time of the tube is requiredto charge the capacitance when an A.C. signal is impressed on thecontrol grid, thus preventing a change in the shielding action of thebalancing electrode until after the electron current to the anode hasincreased beyond the minimum required to establish the arc. Increasedelectron flow due to an A.C. signal on the control grid divides betweenthe resistance and capacitance paths, and consequently the voltage dropacross the resistance is not as great (and thus the balancing electrodeis not driven as far negative) as when E varies gradually.Alternatively, an A.C. firing signal of suificient magnitude impresseddirectly on the balancing electrode immediately increases the anodeelectron current above the minimum required to start the arc. Thus theA.C. signal may be applied to either electrode while the tubecompensates for, and remains relatively insensitive to, DC. drift. I

The various features of the invention will be more clearly understoodfrom the following detailed description in connection with theaccompanying drawings in which:

FIG. 1 is a perspective view of one embodiment of a discharge tube madein accordance with the invention with a portion of the enclosing vesseland the anode broken away to show the internal elect-rode structure;

FIG. 2 is a diagram of the firing circuit of a radio proximity fuzeutilizing the novel tube;

FIG. 3 is a diagram of a circuit alternative to that 0 FIG. 2; and

FIG. 4 is a typical control characteristic curve of the novel tube.

Referring to the drawings, a gas-filled grid-controlled discharge tubeaccording to the invention is provided with the usual subminiature typeradio tube glass envelope 10 containing an ionizable gaseous medium, forinstance, one of the rare inert gases exemplified by neon and argon, ormercury vapor. The envelope 10 terminates in a press 11 in which aresealed both'the lead-in conductors for the electrodes andupwardly-extending rigid rods 12 adapted to support the electrodestructure. Each rod 12 fits into a depression '13 provided in, andengages the outer surface of, a metallic cylindrical anode 14. The rods12 and a lead-in conductor 16, which conductor engages and providespartial support for the anode 14, extend through snug apertures providedin circular discs 17 and 18 of suitable insulating material, such asmica, in planes transverse of the axis of the cylindrical anode 14.Another lead-in conductor 20 extends through the press 11 and throughapertures in the discs 17 and 1 8, and a hook 21 fastened to the upperend of the leadin conductor 20 above the insulating disc 18 supports afilament cathode 23 which extends through apertures in the discs 17 and18 along the axis of the enclosing vessel 10. A separate lead-inconductor 26 extending through the press 11 connects to the lower end ofthe cathode 23 below the insulating disc 17. The filament cathode 23 iscoated along the length between the insulating discs 17 and 18 withthermionically-active material such as barium and strontium oxides toinsure a copious supply of electrons.

A rigid rod 27 and a lead-in conductor 28 extending upward from thepress .11 and through apertures in the insulating discs *17 and 18engage a generally tubular metallic control grid B1 disposed coaxialwith the cathode 23 between. the insulating discs 17 and 18. Theinsulating discs 17 and 18 thus close the upper and lower ends of thecontrol grid 31 and also fixedly position the lead-in conductors 16 and28 and the rods 12 and 27 to. maintain the anode 14 and the control grid31 coaxial with the cathode 23. By the Word tubular in this connectionis meant any form having a substantially perimetrically complete contourwhether such contour be circular or noncircular. In the preferredembodiment of the invention illustrated in the drawings, a rectangularcontour is utilized. A plurality of apertures 33 are provided in thegrid 31 aligned along the length, of the cathode 23.

My invention is particularly concerned with a balancing electrode 34positioned in front of the apertures 33 in the path of electronic flowbetween the cathode 23 and the anode 14. The balancing electrode 34comprises two spaced rigid rods 35 disposed between the control grid 31and the anode 14 and extending parallel to the cathode 23 throughapertures in the insulalting discs 17 and 18. One rod 35 is an extensionof a lead-in conductor extending through the press 11 and is soldered tothe second rod 35 below the insulating disc'17.

While the particular structure of the. tube as illustrated in thedrawings is the preferred embodiment of the invention, other elements ofthe electrodes relative to one another, as regards spacing andconfiguration, which perform the functions explained are within thescope of the invention. It is obvious that the balancing electrode maycomprise a single, or any desired number, of rigid rods 35 in the pathof electron flow between the anode and cathode.

The utility of such a gaseous device is shown in the circuit of: FIG. 2,which represents a conventional firing circuit of a radio proximityfuze. The. electrodes of the tube in FIG. 2v bear the same referencenumerals as those of FIG. 1 and are schematically represented as thougha horizontal sectional view were taken through the electrode structure.After a projectile containing a radio fuze is fired, a source offilament voltage is connected to the cathode 2 3 and a source of anodepotential of approximately 90 volts positive with respect to the cathode23. is connected to the anode 14v through an arming resistor 40,conventionally from the A and B sections respectively of, a deferredaction type battery (not shown) contained; the. fuze. An, armingcondenser 41 connected to the anode 14 is in series with an electricdetonator 44 to, ground. The arming condenser 41 pro-- vides, the energyto fire the electric detonator 44 when the discharge device of theinvention fires and conducts current. The arming resistance 40 and thearming condenser 41in combination provide a time delay to prevent thebursting charge from exploding until the projectile is. well away fromthe gun and from, firing personnel. In order to prevent breakdown of thegas-filled tube of the invention in. the absence of an operating signal,a source of bias voltage negative relative to the cathode 23 isconnected. to the control electrode 31. The operating signal from theamplifier (not, shown) of the fuze is coupled. to the control, grid 31through a condenser 43.

Conventionally, the source of negative grid bias for the control grid 31is the C section of the deferred action type battery of the fuze. Ashereinbefore explained, this C section is subject to decay in outputvoltage during the flight of the projectile, and in order to eliminateany possibility of premature detonation, it was necessary to maintainthe control grid of gas-filled discharge tubes heretofore usedconsiderably more negative than'the starting voltage. Consequently anoperating signal of considerable amplitude was required to overcome thenegative bias and cause discharge of the gas-filled tube.

To provide a gas-filled device which is sensitive to AC. signals and atthe same time prevent any possibility of accidental discharge, thebalancing electrode 34 is connected through the parallel arrangement ofa resistor 46 and a condenser 47 to a source of potential positiverelative to the cathode .23. For every anode potential the dischargedevice of the invention has a critical fiash point function (i.e.,control characteristic) of control grid voltage E and balancingelectrode voltage E A- typical control characteristic curve 50 for agiven anode voltage is shown in 'FIG. 4 of the drawing plotted withvalues of E, as abscissae and values of E as ordinates. Any combinationof E, and E that determines a point above and to the right of the flashpoint function 50 indicates that the shielding action of the controlgrid 31 and balancing electrode 34 are insufficient to limit the anodeelectron current to a value below the minimum required to establish thearc; but if the point is determined below the flash point controlcharacteristic 50, the shielding action of these two electrodes issuflicient to limit the anode current below this minimum. That is, thearc starts as electrode potentials 'pass across curve 50 from the lowerleft to the upper right.

The balancing electrode 34 acts to compensate for changes in electronflow due to drifts in the voltage of the control grid C bias source bylimiting the electron current to the anode below the minimum required tostart the are even though E may drift from its preferred negative valueto an excessively low negative value, i.e., drift in a positivedirection, due to a defective deferred action type battery. As E slowlybecomes more positive due to drifts in the voltage of the C bias source,a few electrons emerge through the apertures 33 in the control grid 31and are drawn primarily to the balancing electrode 34.

. Substantially all the electrons flowing through the apertures' 33 arecollected. by the balancing electrode 34 and flow through the resistor46, thereby lowering E by an amount equal to the voltage drop acrossthis resistor 46. The increased shielding action of the balancingelectrode 34 thus prevents increased electron flow to the anode due togradual changes in 13;.

The voltages of the control grid C bias source and the balancingelectrode source are selected so that the combination of E and E arejust below, i.e., on the nonfiash side of the control characteristic 50of the tube. E and E are balancing voltages in that E, is inverselydependent upon E so that the combination of voltages determines pointswhich are on the non-flash side of the critical flash point function 50even though the output voltage of the C" bias source may vary over awide range. The dotted curve 51 in FIG. 4 is drawn through pointsdetermined by such combinations of E and E,;. It will be noted that allpoints on this dotted curve 51 are below, i.e., on the non-flash sideof, the control characteristic 50 for a considerable range of variationof E As E drifts positively it drives E less positive so that both movealong the curve 51.

It is apparent that if means are provided to momenta-rily raise either Eor E i.e., make either more positive, without significantly changing thevoltage of the other, the combination of voltages will then determine aresistor 46 which in turn varies E and thus the shielding eifect of thebalancing electrode 34. If the decrease in E, is gradual, the increasein electron current is gradual, and the shielding action of thebalancing electrode '34 increases substantially simultaneously withchanges in electron current and prevents the anode current fromincreasing above the minimum required to start the arc. The capacitance47 offers a substantially infinite impedance path to gradual changes ofelectron flow. However, if a rapidly changing signal is impressed on thecontrol grid 31, the sudden increase in electron current does not causean instantaneous drop in B The 500 micromicrofarad condenser 47 offers apath to alternating electron current flow of decreasing impedance withincrease of frequency. This impedance decreases below the 20 megohms ofthe resistance -47 at frequencies above 50 cycles. The increase inelectrons collected by the balancing electrode 34 is momentarilyabsorbed in charging the condenser 47, thus providing a momentary delayin the decrease of E, until the electron flow to the anode 14 increasesbeyond the minimum required to ionize the gaseous medium and therebydistort the electric field and cause the tube to discharge. The timedelay provided in changing E, is greater than the ionization time of thetube. The alternating electron flow divides between the parallelresistance and capacitance paths, and the magnitude of the currentthrough each path is dependent upon the frequency. The voltage dropacross the rmistance 46 is thus never as great, and the balancingelectrode 34 is thus never driven as far negative, as when comparativelygradual changes of voltage are impressed on the control electrode 31. Asource of positive potential for the balancing electrode 34 mayconveniently be obtained from a tap on the B voltage section of thedeferred action type battery.

7 FIG. 3 illustrates an alternative radio fuze firing circuit utilizingthe novel tube of the invention and characterized by a high impedanceinput. A source of bias voltage in the C section of the deferred actiontype battery is connected to the control grid 31 in exactly the samemanner as in the circuit shown in FIG. 2, but the operating signal isimpressed through a coupling condenser 48 onto the balancing grid 34instead of onto the control grid 31. Changes in voltage drop across theresistor 46 vary E and thus compensate for changes in electron currentfiowdue to drifts in the output voltage of the C bias source in exactlythe same manner as hereinbefore explained for the circuit of FIG. 2.Likewise, changes in the shielding action of the balancing electrode 34balance changes in E and the combination of voltages limits the electroncurrent to the anode below the minimum required to start the arc over acomparatively wide range of drift of the C bias voltage source inexactly the same manner as hereinbefore explained. However, it isunnecessary to provide a time delay in the change of the shieldingeffect of one of the electrodes in order to fire the tube on arelatively weak A.C. operating signal. The firing signal is impressedthrough the coupling condenser 48 directly onto the balancing electrode34. When E, is driven positive by an AC. firing signal of sufiicientamplitude, the electron current to the anode is immediately raised abovethe minimum required to establish the arc. Peak values of the AC. signalrequired to start the are are represented by the vertical distancebetween the dotted curve 51 and the control characteristic 50 in FIG. 4of the drawing. It is apparent that the C bias voltage source may varyover a voltage range many times larger than the AC. voltage required tofire the tube without causing accidental detonation. E is inverselydependent upon E and gradual changes in E are compensated for, orbalanced, byvarying E and thus the shielding action of the electrode 34.Rapidly varying signals impressed on the balancing electrode 34immediately raise the anode current above the minimum required toestablish the arc.

It has been found that satisfactory compensating action is obtained witha source of potential connected to the balancing electrode 34considerably less positive than the anode potential, e.g., over a rangeof voltages of from 16 to volts positive relative to the cathodedependent upon the anode potential and the value of the resistance 46.For the application herein described it has been found desirable to biasthe control grid 31 nega tive relative to the cathode, although bydecreasing the spacing, the size of the grid apertures 33, and thenumber of rods 35 in the balancing electrode 34 it is possible toconstruct positive-grid-potential discharge device, i.e., a tube whichprevents the establishment of the are even though a source of potentialpositive relative to the cathode is connected to the control grid 31.The control characteristics of such a discharge device are more towardthe positive region than the typical control characteristic curveillustrated in 'FIG. 4. With the circuit of 'FIG. 3 operated at an anodepotential of 90 volts and with a positive potential of 16 voltsconnected through a resistance of 20 megohms to the balancing electrode34, it has been found that the negative grid bias on the control grid 31may drift positive as much as 8 volts without establishing the are, yetif an AC. signal of 0.15 volt R.M.S. is impressed on the balancing grid34 through the condenser 48, the discharge device is triggered. With thecircuit of FIG. 2 operated at similar voltage values for the anode 14and for the source of potential for the balancing electrode 34, thecontrol grid voltage may drift in a positive direction as much as fivevolts without firing the tube, and yet if an AC. signal of only 0.5 voltR.M.S. is impressed on the control grid 31, the discharge devicedefinitely fires.

' It is apparent from the foregoing description and the drawing that agas-filled discharge device has been described that is substantiallyimmune to drifts in control grid bias voltage and sensitive to weak A.C.operating signal voltages. While I have indicated only the preferredembodiments of the invention, it will be apparent that the invention isby no means limited to the exact forms illustrated or to the useindicated, but that many variations may be made in the particularstructure used and the purpose for which it is employed withoutdeparting from the scope of the invention as set forth in the appendedclaims.

I claim:

1. LAD electron discharge device comprising an enclosing vesselcontaining an ionizable medium, a cathode extending along the axis ofthe vessel, a tubular control electrode coaxial with and surroundingsaid cathode and provided with a plurality of apertures aligned alongthe length of said cathode, a balancing electrode consisting of at leastone rod parallel to said cathode and in front of, and in the path ofelectronic flow through, said apertures, a cylindrical anode surroundingand in substantial parallelism with said cathode, said controlelectrode, and said balancing electrode, insulating disks extending inplanes coincident with the ends of said grid and centering said cathodecoaxially with said grids, and a plurality of lead-in conductorsextending through said vessel respectively from said anode, cathode,control electrode and balancing electrode.

2. A circuit for an electron discharge device in accordance with claim 1adapted to be triggered by an AC. signal, comprising a source ofpotential for said anode, a source of bias voltage for said controlelectrode, and a source of positive potential connected through a highresistance to said balancing electrode of low enough voltage for thecombination of said anode and said bias potentials to preventsuflicien-t electron flow to said anode to establish a discharge are,whereby changes in the number of electrons collected by said balancingelectrode due to slow variations in said bias voltage change the voltagedrop across the high resistance, and thus the shielding action of thebalancing grid, sufficiently to maintain the electron current to theanode below the minimum required to establish the arc, and meansincluding a condenser connected to. said balancing electrode forproviding a time delay in the change of the voltage of said balancingelectrode when an A.C. signal is impressed upon said control electrode,said time delay being of greater duration than the ionization time ofsaid electron discharge device.

3. A circuit for a gas-filled electronic discharge tube adapted to betriggered by an A.C. signal, comprising a gas-filled tube having ananode, a cathode, and two control grids, one grid being an aperturedshield between the cathode and the anode and the other grid being abalancing grid generally aligned between the aperture and the anode, asource of potential for said anode, a source of bias voltage for saidshield grid, means for applying an A.C. signal to said shield grid,means in cluding a source of positive potential connected to saidbalancing grid through a high resistance 'and a capacitance alsoconnected to said balancing grid for compensating only for changes inelectron current due to gradual variations in the voltage .of said biassource, whereby variations in electron current due to an A.C. signalonsaid control 'grid will not be compensated for.

4. A circuit for a gassfilled electronic discharge tube adapted to betriggered by an A.C. signal, comprising a gas-filled tube having ananode, a cathode, and two control grids, one grid being an apertur edshield between. the cathode and the anode and the other grid being abalancing grid generally aligned between'the aperture .and the anode,means for applying a potential to the anode positive relative to thecathode, means for applying a D.C.bias voltage to said shield grid,means for applying to the, balancing grid through a high resistance avoltage positive relative to the cathode, whereby said balancing gridassumes a voltage depending inversely on the grid current through thehigh resistance which current in turn is greater for more positivevalues of said bias voltage, and means for applying an A.C. signal tosaid balancing grid.

5.. A circuit for a gas-filled electronic dischange tube adapted to betriggered by an A.C. signal comprising a gas-filled tube having ananode, a cathode and two control grids, one grid being an aperturedshield between the cathode and the anode and the other grid being abalancing grid in the path of electronic flow through said aperture, asource of potential for said anode positive relative to said cathode,means for applying to said' shield grid a bias potential E means forapplying to said balancing grid through a high resistance a potentialpositive relative to the cathode, whereby said balancing grid assumes apositive voltage E depending inversely on the flow of electron currentthrough the high resistance which current is greater for more positivevalues of E said tube having a critical flash point function of E sand Efor said anode potential wherein all values of E and E .on the nonflashside of said function prevent the establishment of a discharge are,potentials E, and E being balancing potentials on said nonflash side ofsaid function and remaining so for gradual changes of E and means. forapplying said A.C. signal to one of said grids with-, out significantlychanging the potential of the other grid.

6. An electron discharge device comprising an enclosing vesselcontaining an ionizable medium, a cathode, a control electrodesurrounding the cathode except for atleast one aperture, a balancinggrid in front of, and in. the path of electronic flow througln saidaperture, and a cylind-rical anode encompassing said control electrodeand said,

balancing electrode, whereby said balancing electrode when connected toa voltage positive relative to said cathode tends to collect electronsflowing through said aperture. I

7. A circuit for an electron discharge device in accordance with claim 6adapted to be triggered by an A.C. signal, comprising means for applyinga potential to said anode positive relative to the cathode, means forapplying to said control electrode a DC. bias potential E means forapplying tov the balancing electrode through a high resistance apotential positive relative to the cathode whereby said balancingelectrode assumes a positive value E depending inversely on the electroncurrent I flowing through the high resistance which 1,, in turn isgreater for more positive values of E said tube having a controlcharacteristic curve for said anode potential with E and 15,, ascoordinate axes wherein all combinations of E and E which determinepoints to the left of the curve are insuflicie'nt to establish adischarge arc, said E and E being balancing voltages just on said leftside of said curve and remaining so for gradual change in E and meansfor applying said A.C. signal to one of said grids without significantlychanging the potential of the other grid.

References Cited in the file of this patent UNITED STATES PATENTS1,921,004 Samuel Aug. 8, 1933' 1,995,176 Glaser u Mar. 19, 19352,061,254 Rockwood NOV. 17, 1936 2,460,794 Selvidge Feb. 1, 19492,491,425 Stutsman Dec. 13, 1949

